The present invention generally relates to contaminant removal and, more particularly, to apparatus and methods of contaminant removal employing gas-liquid contact and separation.
It is of great interest to control and limit the concentration of carbon dioxide (CO2) in occupied spaces, including homes, buildings, transportation vehicles, aircraft and spacecraft. It is particularly important to control CO2 concentrations in enclosed vehicles like aircraft or spacecraft. In aircraft, fresh air enters the occupied space as bleed air from the engine, and results in increased fuel consumption. Decreasing the bleed air flow would improve fuel efficiency, but would require a technology to remove CO2 from the air. The Federal Aviation Administration (FAA) of the United States limits the acceptable concentration of CO2 to 5000 ppm, while aircraft typically have 1500-2300 ppm. In spacecraft, no fresh air is available, and the cabin air must be preserved in a healthful condition.
Crews of the International Space Station (ISS), with elevated CO2 levels just under 4 torr (5300 ppm), have reported symptoms such as early fatigue onset, impaired function and decision-making, and headaches. Law J, Alexander D (2016). CO2 on the International Space Station: An Operations Update. Annual AsMA Meeting; 24-28 Apr. 2016; Atlantic City, N.J., USA; https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150019624.pdf. Long duration, deep space missions lengthen crew exposure to these conditions. NASA is requiring future spacecraft to maintain CO2 partial pressures in the vessel atmosphere below 2 torr (2600 ppm) to preserve crew health, and maintain alertness and comfort. Therefore, more advanced CO2 removal systems are required for next generation deep space vehicles in order to maintain a much lower CO2 partial pressure. In addition, deep space vehicles are required to have a lower size, weight, power, and thermal load, and use fewer consumables, while fixing existing safety problems that are apparent in current systems. The maintenance interval of current systems (three to six months) is also required to jump to three years.
CO2 recovery and recycling is a critical component of the air revitalization system for long duration missions. Presently on ISS, the carbon cycle, or carbon loop, is not closed and CO2 is either discarded to space or processed through a Sabatier reactor to recover water; methane produced by the Sabatier reactor is discarded to space. Longer duration missions will require a more closed carbon loop to minimize carrying disposable resources in the vessel—such as water, hydrogen, oxygen, etc.—that might otherwise be replenished from recycling CO2. In other words, any discarded carbon dioxide increases the amount of oxygen or water required to be brought with the mission.
For applications in spacecraft or aircraft, the size and weight of the overall system must be minimized. Great emphasis must be placed on minimizing the size, weight and number of scrubber or stripper modules. It is well known that stripper modules can be made to be more efficient if a sweep gas is used to flush the permeate out of the module, and that this can minimize the size, weight and number of such modules. But the source of this sweep gas is also important, since if it is foreign to the process then a supply of this gas must be provided.
Such a closed-loop CO2 recovery system should be capable of throttling its process when process demand is lower in order to reduce energy consumption. For missions to Mars, some plans include landing on the planet and remaining there for eighteen months, during which the Mars Transfer Habitat remains in Mars orbit, unoccupied. The life support system would remain in an operationally ready state during this period able to resume operation with high reliability for the return flight quickly.
In the past, solid adsorbents have been used for CO2 removal. However, liquid absorbents have significant advantages over solid adsorbents. The ability to pump the absorbent from scrubber to stripper stages allows for continuous absorption and regeneration of the sorbent, which is generally more stable and reliable than alternating adsorbent beds between absorption and regeneration, and eliminates the need for a complicated valve network. Liquid may also be easily replenished or exchanged without disassembly.
Existing state-of-the-art CO2 removal systems include the Carbon Dioxide Removal Assembly (CDRA) aboard the ISS, which relies on solid zeolite adsorbents that experience a particulate dusting problem and is higher in size, weight and power when compared to estimates of a liquid system. Other CO2 removal systems include amine-based systems like those used on submarines. These amines are prone to outgassing of dangerous and odorous products, air oxidation and thermal degradation, and can be corrosive.
The above facts suggest a great need for improved apparatus and methods to remove contaminants from supply air in environments such as deep space vehicles.